1. Field of Invention
The present invention relates to a variable valve lift apparatus, and more particularly, to the variable valve lift apparatus that can achieve the improvement of fuel consumption, rapid and accurate control of valve lift change, weight and cost reduction, and the improvement of dynamic characteristic and durability of valves.
2. Description of Related Art
In general, variable valve lift technology is being researched to improve driving performance and fuel consumption by changing the size of the valve lift according to the driving conditions of an engine.
FIG. 1 through FIG. 3 are drawings which show the structure and the operational principle of a variable valve lift apparatus according to a prior art. FIG. 1 is a perspective view of a cam follower of a variable valve lift apparatus according to a prior art. FIG. 2 is a cross-sectional view along a line I-I of FIG. 1 for explaining the operational principle of a variable valve lift apparatus according to a prior art.
Referring to FIG. 1 and FIG. 2, the cam follower comprises a lost lever 1, a locking pin 2, an inner arm 3, a lost motion spring 4, and a hinging pin 5. The lost lever 1 includes two lost lever pads 6. The lost lever 1 is the external body of the cam follower. By driving force through the rotation of a high lift cam moves the lost lever 1, which is mounted at the outside of the inner arm 3 and connected with the inner arm 3 by the hinging pin 5.
The inner arm 3 includes a bearing shaft and a roller 7. The inner arm 3 is the internal body of the cam follower. Inside the inner arm 3 is the locking pin 2 installed, which can be moved forward by hydraulic lash adjuster 8. The lost motion spring 4 is fixedly mounted between fixing plates of the lost lever 1 and the inner arm 3 and functions as an actuator making the lost lever 1 return to its original position after its motion relative to the inner arm 3.
Hereinafter, the principle of low lift and high lift operation of a variable valve lift apparatus according to a prior art will be explained.
Firstly, the principle of low lift valve operation of the variable valve lift apparatus is as follows.
The roller 7 is generally a rolling bearing and has the function of delivering the driving force through the rotation of a low lift cam to the inner arm 3 through bearing, shaft fixedly installed in the inner arm 3 by making rolling contact with the low lift cam. At the moment, the driving force through the rotation of the low lift cam acts on the roller 7 with a contact portion of the inner arm 3 and the hydraulic lash adjuster 8 functioning as an axis of rotation and thereby the hinging pin 5 descends.
Therefore, in case the low lift cam makes rolling contact with the roller 7, the driving force through the rotation of the low lift cam is delivered to a valve stem 9 connected with the hinging pin 5 of the inner arm 3 and thereby a valve operates in low lift. In this case, a high lift cam spins the lost lever 1 with no effect on a valve. This motion of the lost lever 1 is called lost motion.
The operational principle of the lost motion of the lost lever 1 will be described below. The lost lever pad 6 is not a rolling bearing and is a portion making friction contact with the high lift cam. It has the function of delivering the driving force through the rotation of the high lift cam to the lost lever 1 and making the lost lever 1 rotate on the hinging pin 5. The rotation is the motion relative to the inner arm 3.
If the driving force through the rotation of the high lift cams is applied to the lost lever pads 6 with the locking pin 2 received inside the inner arm 3, the rotation of the lost lever 1 becomes lost motion. That is to say, the rotating motion applies no force to the cam follower and the lost lever 1 comes back to its original position by the elastic force of the lost motion spring 4 (Refer to FIG. 2). In this case, only the driving force through the rotation of the low lift cam is applied to the hinging pin 5 through the roller 7 and thereby a valve operates in low lift.
Next, the principle of high lift valve operation of the variable valve lift apparatus is as follows.
In case the locking pin 2 is moved forward with the increase of the hydraulic pressure by the operation of the hydraulic lash adjuster 8, the rotating motion of the lost lever 1 is locked by the locking pin 2. At this moment, the hinging pin 5 rotates down on a contact portion of the hydraulic lash adjuster 8 by the driving force through the rotation of the high lift cam being applied to the lost lever pads 6. Accordingly, a valve operates in high lift through the valve stem 9 connected with the hinging pin 5.
FIG. 3 is a drawing which compares the principles of low lift valve operation and high lift valve operation of a variable valve lift apparatus according to a prior art. Referring to FIG. 3, foregoing explanations will be organized hereinafter.
In case hydraulic pressure of the hydraulic lash adjuster 8 is lower than the load of a spring supporting the locking pin 2, a variable valve lift apparatus operates in low lift condition. In this case, the rotating motion of the lost lever 1 by a high lift cam becomes lost motion.
This is because the lost lever 1 operates in a state of being unlocked with the locking pin 2 received inside the inner arm 3 on account of the locking pin 2 being supported by the spring. In other words, the high lift cam spins with no effect on a valve while the low lift cam makes rolling contact with the roller 7 and thereby the valve operates in low lift.
In case hydraulic pressure of the hydraulic lash adjuster 8 is higher than the load of the spring supporting the locking pin 2, a variable valve lift apparatus operates in high lift condition. In this case, the locking pin 2 moves forward and protrudes from the inner arm 3 by overcoming the load of the spring through higher hydraulic pressure.
Accordingly, the lost lever 1 becomes in locked condition, the driving force through the rotation of the high lift cams acts on the lost lever pads 6 with a contact portion of the hydraulic lash adjuster 8 functioning as a pivot, and thereby the hinging pin 5 descends and a valve operates in high lift. At this moment, the low lift cam and the roller 7 don't make contact with each other in principle as FIG. 3 shows.
This is because the high lift cams make contact with the lost lever pads 6 on the point of the low lift cam making contact with the roller 7 and the inner arm 3 descends in high lift condition.
The problems of the prior art includes at least the followings.
In high lift condition, the valve lift operation is carried out not through a rolling bearing but through the lost lever pads 6. As a result, friction loss is large, abrasion happens, and thereby durability of a variable valve lift apparatus is deteriorated.
In low lift condition as well as high lift condition, two high lift cams always make friction contact with the lost lever pads 6. Therefore, friction loss continues while valves keep opening or closing.
In the meantime, even in high lift condition the contact of the low lift cam and the roller 7 may actually happen and thereby the gap between the high lift cams and the lost lever pads 6 may happen.
This is because even though machining is executed so precisely it's almost impossible for the high lift cams and the low lift cam to simultaneously come into contact with the lost lever pads 6 and the roller 7 respectively. This kind of phenomenon may result from defects of machining precision and vibrations of the variable valve lift apparatus, etc.
In case the gap is formed, lift loss happens in high lift condition, impulsive load acts on the valve apparatus, and thereby durability may be deteriorated. In addition, the structure and the shape of a variable valve lift apparatus according to a prior art are complicated and there are lots of parts in it. This becomes the cause of rise of manufacturing cost and exerts a bad effect on dynamic characteristic and durability of the variable valve lift apparatus.
A variable valve lift apparatus according to a prior art operates through oil pressure of oil gallery by controlling an oil control valve (OCV). On this account, the variable valve lift apparatus is sensitive to oil temperature and pressure and an unnecessary demand for increasing a capacity of hydraulic pump may arise.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.